A switching power supply is a power supply device using a switching element to convert and regulate electrical power in a power conversion device for obtaining a desired output power from an input power. The switching power supply includes a power supply device including a DC-DC converter that converts a direct-current power to another direct-current power, and a direct-current power supply that includes a rectifying device to convert an alternating-current power to a direct-current power and supplies the direct-current input power to the DC-DC converter.
As a use of the switching power supply, it is already known that an LED lighting device having high circuit efficiency is obtained by turning on a light-emitting diode. For example, a DC-DC converter includes a step-down chopper, and an increasing current flows through an inductor via a switching element in an on state, so that a voltage is induced in a secondary winding magnetically coupled thereto. The voltage is fed back and the on operation of the switching element is continued. Besides, a resistance element for detecting the increasing current is inserted in series to the switching element, and a control circuit is added to turn off the switching element when the drop voltage of the resistance element exceeds a previously set threshold.
In the above structure, when the increasing current exceeds the threshold and the switching element is turned off, electromagnetic energy stored in the inductor is released, and a decreasing current flows via a diode and an output capacitor of an output end. When the decreasing current becomes 0, the switching element is turned on by counter electromotive force generated in the secondary winding of the inductor. The DC-DC conversion by self-excitation constant-current control is performed by repeating the circuit operation, and the light-emitting diode is turned on.
On the other hand, a semiconductor element using a wide band gap semiconductor or a compound semiconductor, such as III-V group semiconductor and IV semiconductor, having a large band gap, for example, silicon carbide (SiC), gallium nitride (GaN) or diamond attracts attention as a semiconductor element having the potential to greatly break through the performance limit of a Si power device. That is, in high speed switching, high temperature operation, large power operation and the like, the semiconductor element can operate in a region where a Si or GaAs element cannot operate. Thus, also in the field of a high frequency power device, the expectation for the semiconductor is high. Here, the wide band gap semiconductor is a semiconductor having a band gap wider than gallium arsenide (GaAs) having a band gap of about 1.4 eV. The wide band gap semiconductor is, for example, a semiconductor having a band gap of 1.5 eV or more, gallium phosphide (GaP, band gap: about 2.3 eV), gallium nitride (GaN, band gap: about 3.4 eV), diamond (C, band gap: about 5.27 eV), aluminum nitride (AlN, band gap: about 5.9 eV), silicon carbide (SiC) or the like.
A wide band gap semiconductor transistor is an element which can be used as a switching element, and more specifically, is a transistor fabricated using a wide band gap semiconductor. For example, the wide band gap semiconductor transistor is a field effect high-frequency transistor including a pair of main terminals (drain, source) and a control terminal (gate). The transistor is typically a high electron mobility transistor (HEMT). Since the semiconductor transistor has the excellent characteristics as described above, when the wide band gap semiconductor transistor is used as a switching element of a switching power supply, a high frequency operation of 10 MHz or higher can be performed. As a result, the switching power supply, especially an inductor can be greatly miniaturized.
Incidentally, as the wide band gap semiconductor transistor, a JFET (junction FET), a SIT (Static Induction Transistor), a MESFET (metal-semiconductor FET: Metal-Semiconductor-Field-Effect-Transistor), a HFET (Heterojunction Field Effect Transistor), a HEMT (High Electron Mobility Transistor) and a storage FET are enumerated.
Besides, the wide band gap semiconductor transistor often has a normally-on characteristic in which a drain current flows when a gate voltage is 0. Accordingly, in order to certainly turn off the semiconductor element having the normally-on characteristic (hereinafter referred to as a normally-on switch), a drive circuit for negative gate voltage is required. Incidentally, a wide band gap semiconductor transistor having a normally-off characteristic is also obtained. In this case, a drive circuit for positive voltage is required.
When constant-current control is performed, a related art switching power supply requires a current feedback type feedback circuit including an impedance unit, such as a resistance element, that is inserted in series to a switching element and detects an increasing current flowing through an inductor, and a control circuit that turns off the switching element when the voltage drop of the impedance unit reaches a previously set threshold. Thus, the circuit structure becomes complicated, and miniaturization is difficult.
When the wide band gap semiconductor transistor is used as a switching element of a switching power supply, since the switching element can be operated at a high frequency of 10 MHz or higher as stated above, the whole apparatus can be remarkably miniaturized. In this case, when a constant current element including a wide band gap semiconductor transistor is connected in series to the source of the wide band gap semiconductor transistor, an impedance unit such as a resistance element for current detection can be omitted, and a structure of a gate drive circuit is simplified, and therefore, further miniaturization can be realized.
Although the switching power supply using the wide band gap semiconductor transistor can be further miniaturized by using the two wide band gap semiconductor transistors for switching and for constant current control as stated above, there is a limit in meeting a request for further cost reduction and miniaturization.
An exemplary embodiment provides a switching power supply device in which cost reduction and miniaturization are realized by using one switching element for switching and for constant current control, and a lighting apparatus using the same.